(1) Field of the Invention
The present invention relates to a semiconductor device having a so-called three-dimensional capacitor, and more particularly relates to a semiconductor device using a noble metal for a conductive film constituting an electrode of the three-dimensional capacitor and a method for fabricating the same.
(2) Description of Related Art
Capacitor insulating films made of a high-dielectric-constant material or a ferroelectric material of a perovskite crystal structure are used for semiconductor memory devices such as dynamic random access memories (DRAM) and ferroelectric random access memories (FeRAM), and need to be subjected to heat treatment at a relatively high temperature in an oxygen atmosphere in order to provide crystallization and improve their quality. If, during this heat treatment, the composition of a capacitor insulating film is shifted from its design value, for example, because of a reaction between the capacitor insulating film and an electrode material, this causes deterioration in characteristics such as a decrease in the polarization of the capacitor insulating film and an increase in leakage current. To cope with this problem, platinum (Pt) or ruthenium (Ru) both with extremely poor chemical reactivity is generally used for a conductive film material which forms a lower electrode or an upper electrode to contact such a capacitor insulating film. This provides resistance to oxidation and can prevent the composition of the capacitor insulating film from being shifted.
With the increasing miniaturization and integration of semiconductor integrated circuits, memory cells of DRAM devices and FeRAM devices have also been demanded to be finer. As a result, capacitive elements constituting memory cells are being formed in three-dimensional shapes to increase their capacitances per unit area.
For example, Japanese Unexamined Patent Publication No. 2001-160616 (hereinafter, referred to as Document 1) discloses an example in which a lower electrode containing platinum or iridium is formed along the top of an underlying substrate of an uneven shape in cross section by metal organic chemical vapor deposition (MOCVD) and then a dielectric film and an upper electrode are successively formed on the formed lower electrode, thereby forming a capacitor. With this method, the lower electrode can be formed, even on a large-aspect-ratio groove (concave part), to have a good coverage and a uniform thickness.
Japanese Unexamined Patent Publication No. 2002-231905 (hereinafter, referred to as Document 2) discloses an example of a capacitive element including a high-dielectric-constant film or a ferroelectric film of a three-dimensional shape, for example, a concave shape. In this example, a lower conductive film is formed along a recess by sputtering and then an upper conductive film is formed by CVD on the formed lower conductive film. Thus, an electrode film consisting of the lower conductive film and the upper conductive film is formed to prevent the electrode film from being disconnected at the bottom corners of the formed electrode film. In this way, the use of sputtering improves the lower conductive film in its morphology, and the later use of CVD provides a uniform thickness of the upper conductive film. Hence, the lower and upper conductive films are difficult to agglomerate even during heat treatment for crystallizing the dielectric film. As a result, the electrode film can be prevented from being disconnected at its bottom corners.
Japanese Unexamined Patent Publication No. 2001-223345 (hereinafter, referred to as Document 3) discloses an example in which a glue layer is formed on the sidewalls of a lower electrode of a concave cross section, thereby preventing the lower electrode from peeling off during heat treatment for crystallizing a dielectric film. In this case, the glue layer consists of an oxide using any one of titanium (Ti), tantalum (Ta), tungsten (W), and copper (Cu) or a mixture of such an oxide and another metal. Document 3 also discloses that a compound consisting of barium strontium ruthenium oxide ((Ba, Sr)RuO3) or an amorphous material containing ruthenium (Ru) or oxygen (O) is used for the glue layer. Furthermore, the bottom electrode is composed of ruthenium (Ru), ruthenium oxide (RuO2) or a mixture of them.
Japanese Unexamined Patent Publication No. 2002-76306 (hereinafter, referred to as Document 4) discloses an example in which a glue layer of tantalum nitride (TaN) is formed between a metallic film of a platinum group constituting a bottom electrode of a capacitive element and an insulating film of silicon oxide, thereby preventing the metal film from peeling off at the interface with the insulating film. Furthermore, in this case, the upper end of the glue layer is removed, and then covered with the bottom electrode, thereby preventing the glue layer from being oxidized.